In modern motor vehicles, at least two different electrical systems, for example a 12 V electrical system and a 48 V electrical system are usually employed, which supply different electrical/electronic components with the electrical supply voltage needed by these. In order to save the energy supply units of the motor vehicle—usually rechargeable batteries—when the motor vehicle is in a switched-off state and consequently cannot be recharged in driving mode with the help of a generator installed in the motor vehicle, the electrical systems that are present in the motor vehicle can be frequently switched between an active state and a so-called idle state. In the idle state, the electrical consumers that are connected to the electrical system can be electrically isolated from the energy supply unit by means of an electrical/electronic switch or by using suitable integrated switching circuits (ICs) in the manner of voltage transformers, communication ICs etc. The idle state is thus characterized by a greatly reduced energy consumption of the electrical consumers that are connected to the electrical system in the motor vehicle, so that the energy supply unit is saved. In particular, undesirable complete discharge of the energy supply unit is prevented in this way, when the vehicle is in the switched-off state.
In the active state, the energy supply unit, by contrast, is connected to the electrical system assigned to it, i.e. an electrical isolation by means of the interruption circuits mentioned above does not take place so that the energy supply unit can provide the electrical consumers with the electric energy required for nominal operation. In order to now switch the electrical systems from said idle state to the active state, which corresponds to a so-called “wake-up” of the electrical system from the idle state, an electrical/electronic switch, which isolates the energy supply unit from the remaining residual electrical system, is switched to a closed state so that the energy supply unit is electrically connected to the electrical system.
In case that the motor vehicle comprises multiple electrical systems, such a wake-up signal has to be individually transmitted to all electrical systems and each electrical system separately switched from the idle state to the active state. A corresponding activation signal can be provided in all electrical systems of the motor vehicle for example by a CAN or LIN bus system that is available in the motor vehicle, but which however is technically relatively complex to realise since the bus system, in all electrical systems, has to be coupled to the corresponding switches or voltage dividers/ICs. The same applies when instead of the bus system a starter device for activating the internal combustion engine of the motor vehicle, for example the so-called “terminal 15” is activated when switching on the starter device.